Delete the kagane-specific cover path (#63)

The web proxy for kagane covers is dead: since #62 every Site's cover
bytes land in the content-addressed store at creation or on the poll,
and the one public route serves them all. Remove the second way to
reach a Cover:

- Bookmark.CoverURL() and the templates' use of it: templates render
  the wire value (.Cover) and nothing else.
- GET /img/kagane/{id}, web.CoverFetcher, coverIDRe: the route and its
  identifier validation are gone, and with them web/cover.go.
- store.KaganeImageID, GetKaganeCover, PutKaganeCover,
  kaganeCoverSourceURL: the proxy's persistence.
- Bookmark.CoverSource: dead once CoverURL is gone.

Acquisition keeps the browser where kagane genuinely needs it, but the
Site name leaves the routing: kaganeImageURLRe lives in browser.go with
the rest of the per-Site knowledge, browserCoverURL claims the URLs the
sidecar alone can fetch, and fetchCoverBytes routes by URL shape with
no Site argument. No plain-TLS fallback for a claimed URL — that would
only retrieve a challenge page.

Cover tests: kagane route tests removed, the general-route guarantees
they pinned kept and re-pinned — unstored and traversal-shaped
addresses serve nothing (TestPublicCoverRejectsUnknownAddress), non-image
content types are never echoed back (TestPublicCoverNeverEchoesNonImage
+ TestCoverStoreAcceptsAnySourceURL). Store content-addressing and
reopen-persistence tests rewritten against PutCover/GetCover.

No Site name remains in a cover code path outside the acquisition
module; go test ./... green.

backend/internal/latest/acquire.go

@@ -42,8 +42,8 @@ type Acquirer struct {
 	// Covers retrieves the cover bytes. Nil leaves the Cover blank and the
 	// chapter half working.
 	Covers CoverBytesFetcher
-	// BrowserCoverFetch retrieves kagane cover bytes through the browser
-	// sidecar. Nil leaves kagane Covers blank; nothing falls back to a plain
+	// BrowserCoverFetch retrieves browser-claimed cover bytes through the
+	// sidecar. Nil leaves those Covers blank; nothing falls back to a plain
 	// fetch, which would only ever retrieve a challenge page.
 	BrowserCoverFetch BrowserCoverFetcher
 	// Ctx cancels in-flight acquisitions at shutdown. A hook signature has
@@ -136,7 +136,7 @@ func (a *Acquirer) acquire(ctx context.Context, sr store.Series) {
 	if !ok {
 		return
 	}
-	bytes, contentType, err := fetchCoverBytes(ctx, sr.Site, cover, a.BrowserCoverFetch, a.Covers)
+	bytes, contentType, err := fetchCoverBytes(ctx, cover, a.BrowserCoverFetch, a.Covers)
 	if err != nil {
 		log.Printf("acquire %q: fetch cover %s: %v", sr.Key(), cover, err)
 		return